The Vietnam conflict saw
widespread use of radar targeting in night or adverse weather
conditions; though well suited to soft-skinned targets, this technique
is limited in accuracy to around 100 feet and thus isn't ideal for
hitting hardened point targets.

Daylight attacks in the final phase of the conflict often
utilised the Paveway laser guided bomb, with a miss distance of several
feet, the weapon targeted by the AN/AVQ-10 Pave Knife TV/laser pod.

The USAF was more than pleased with the performance of Pave
Knife, but was very unhappy about the inherent daytime only limitation
of the system. This led to the development of Pave Tack, an infra red
imaging laser target designator.

Ford Aerospace AN/AVQ-26 Pave Tack

Pave Tack was initially destined to equip the F-4E, RF-4C,
F-111E and F-111F in USAF service, however this has since been altered.
The analogue F-111E did not receive the pod, and the Europe based USAFE
receiving pods only for the F-111F and tactical recce RF-4C. Carried by
an F-4 the pod is pylon mounted, the F-111 however employs a more
sophisticated mounting, that being a retractable cradle mounting in the
fuselage weapons bay.

The Ford Aerospace AN/AVQ-26 Pave Tack was designed with
performance in mind, above all. To that effect the USAF willingly
accepted substantial increases in system weight to get maximum picture
quality; to date Pave Tack has not been matched in that respect.

Weighing 1280 lb, Pave Tack may be physically divided into
two
sections, a base section and head section (please see diagram). The
head
section is comprised of a roll turret with a �92 degree roll angle, the
turret constraining the actual FLIR head which may pitch from O to 180
degrees.

As a result the pod may view the entire lower hemisphere and
small portions of the upper hemisphere.

The FLIR head itself contains several systems, most
significantly the Texas Instruments AAQ-9 FLIR assembly (see TE March
'84).

The AAQ-9 operates in the 10 micron IR band, with dual fields
of view of 7.5 and 2 degrees, with further 2X magnification if selected.

The FLIR window then mounts a yaw stabilised yoke, that in
turn mounting a pitch stabilised mirror. The mirror is used to
stabilise
both the FLIR picture and the laser beams, that with an accuracy around
1 / 1000 degree, reference provided by gyros.

The AVQ-25 laser designator/ rangefinder mounts on either
side
of the FLIR, the transmitter with its 1.06 micron solid state Nd:YAG
optically pumped pulse mode laser and beam expanding/pulse modulating
optics on the left, the receiver with its beam focusing optics and
solid
state avalanche diode optical detector on the right.

Laser range is classified; the range error in rangefinding
has
been described as 10 ft rms. The laser range is though, far less than
the 10 nm+ detection range of the FLIR, against large targets.

Both the FLIR and laser equipment see the outside world
through a large IR window, with two glass viewing ports for the laser
systems. The base section of the pod drives the turret with a motor
(just as the turret drives the head) and ring gear, both motors driven
by power amplifiers mounted in the aft of the base section.

Divided into three areas, the aftmost of which also contains
the FLIR electronics, power supply and autotracker, the base section is
quite densely packed. The central area houses the laser power supply,
system power supply, pod control electronics (both operating mode
control and stabilisation) and last but not least the video recorder,
which turns Pave Tack into both a damage assessment camera and all
weather photorecce tool.

The forward area of the pod then contains the environmental
control systems and the pod's digital computer. The computer is a
16-bit
machine with a modest 32 kbyte of nonvolatile magnetic core memory,
which is adequate for built-in-test tasks and the pod's internal
housekeeping functions. In RAAF F-111Cs, which are devoid of digital
computers, this machine will carry launch software for both the GBU-15
and particularly the AGM-84A Harpoon (most likely block ID) and will
carry out the necessary launch sequencing tasks.

The pod's systems are very modular, line replaceable units
may
thus be easily replaced at an intermediate level maintenance shop, such
as the unit currently being equipped at Amberley.

As is apparent, the pod is driven with digital
signals-controlling pod aimpoint, operating mode, laser mode and laser
coding, together with the analogue signals controlling the FLIR. This
has created problems for the RAAF, as the F-111C's analogue nav/attack
doesn't have available digital reference signals. This has forced the
need to custom design a system of interface circuits,
providing the necessary analogue/ digital, digital /analogue conversion
and clocking functions to tie the pod into the AJQ-20.

Pave Tack attack profile. Entering the target area,
the
Pave Tack pod is extended and cued onto a known geographical feature.
The laser rangefinder is fired and the computer uses the precise
position data to update the INS. The pod is again cued, now onto the
target as estimated by the nav-attack system. The navigator will then
take a close look at the target, to select the best impact point for
his
laser guided munitions. This could be a bridge support, a powerplant
generator housing, an airbase runway or the command bunker of a SAM
site. The Pave Tack crosshairs are placed on the impact point and the
navigator initiates tracking, manually or with autotrack. At this stage
the pilot will begin a pullup to loft the bomb, as commanded by the
nav-attack system. The Paveway laser guided munition is released at a
preprogrammed angle and continues to climb, having deployed its wings,
along a ballistic trajectory. As this occurs the pilot completes his
manoeuvre, turning away from the target while the navigator continues
to
track the impact point. The laser cannot be fired until the bomb passes
the apex of its flightpath, and starts descending, otherwise the seeker
would fly the bomb in a straight line toward the target and it would
fall short. Once the bomb dives toward the target, the navigator may
fire the laser, the seeker locks on and the bomb guides into the
target. The whole procedure is recorded on the Pave Tack video
recorder,
for damage assessment and analysis. Pave Tack is then retracted and the
F-111 exits the target area (Ford Aeronutronics images).

Rockwell GBU-15. Turning toward a target, this USAF
F-111F
displays its deadly payload of two GBU-15s and their associated Hughes
AN/AXQ-14 datalink pod. GBU-15 isn't cheap, at US$135,000 apiece, but
even so is a cost effective tool when dealing with heavily defended
targets. The USAF currently has well over 700 rounds in stock with an
outlook toward a further increase; Rockwell apparently suggest a unit
cost around US$60,000, given the USAF orders another 1300 rounds. The
complete weapon weighs in at 2450 lb and some sources suggest up to 20
nm of standoff range, for a high speed launch. (via F-111.net)

The Pave Tack pod is mounted in a roll cradle, which in turn
fits into the weapons bay, virtually filling it. The cradle rotates by
180 degrees to either stow the pod inside or extend it for operation,
which takes about 5 seconds, and can be carried out at speeds up to
Mach
1.4. Extended, the pod's drag cuts off 2 nm of range per minute of
extension; retracted its drag is negligible.

The RAAF is acquiring 10 Pave Tack pods, a sensible number in
view of the 12 aircraft strong nominal strike force and high
reliability
of the pods. One would anticipate all aircraft but the RF-111Cs being
fitted with the cradles and interface systems, allowing the selective
fitting of pods to specific aircraft flying specific missions.

Pave Tack has been in operational use with the USAF's 3rd Air
Force since mid 1982, carried by the F-111Fs of the 48th Tactical
Fighter Wing based at Lakenheath in the UK. This significantly
increased
the lethality of the aircraft in the high threat central European
theatre; it is interesting to note that the USAF equipped all of its
European F-models with cradles, maintaining though, a 2.5 to 1 aircraft
to pod ratio, suggesting the use of the pods only on particular
missions.

Pave Tack will prove to be a valuable tool, extending the
effective lifetime of the F-111C's analogue nav/attack until a suitable
digital upgrade is carried out, aside from its inherent use as a
reconnaissance system.

Ford
Aeronutronics image.

Rockwell GBU-15 Cruciform
Wing
Weapon

The GBU-15 modular glidebomb came into being as the result of
the USAF's pressing need for a heavy weight standoff weapon.

Though the Paveway laser guided bomb was effective it did
require line of sight to the target, similarly the HOBOS TV guided bomb
had range limitations. Neither weapon was suitable for low level
standoff release. The GBU-15 was the solution to the problem, conceived
as a modular weapon allowing the user the choice of seeker or warhead
(rather bomb body).

The weapon operates in two distinct modes, direct and
indirect
attack. In a direct attack the operator on board the launch aircraft
views the scene observed by the camera in the bomb's nose on a head
down
display, he then steers a set of crosshairs onto the desired target and
engages a contrast lock. The bomb is then released and glides into the
target.

The weapon's primary mode of operation is however, indirect
attack. To allow this, the bomb is fitted with a datalink receiver and
transmitter, likewise the launch aircraft carries a receiver and
transmitter in a pod. The transmission links use highly jam resistant
transmission techniques, allowing the bomb to transmit a TV picture to
the aircraft and the aircraft a series of commands to the bomb. In
executing an indirect attack, the aircraft lofts the bomb, preferably
from behind a natural obstacle. GBU-15 will then climb and glide along
a
programmed flightpath, its autopilot receiving midcourse updates via
the
datalink, until it reaches the target area. The operator may then steer
the crosshairs onto a target or part of a target, guiding the bomb in
that direction. The weapon may be locked on and use its contrast lock
for the terminal phase, but the operator may also steer the bomb
manually throughout the attack.

Alternately he may choose to change the exact impact point
once the contrast lock is engaged. As is very apparent the weapon is
exceptionally flexible in both delivery and configuration. The latter
aspect is more evident upon closer examination. Physically the GBU-15
is
divided into six modular assemblies. Starting from the nose, we find
the
TV guidance module, containing optics, a stabilised platform mounting a
vidicon and all the electronics for processing the video, together with
the contrast lock. This unit is interchangeable with an infra-red (IR)
unit which allows day/night/adverse weather attacks. The day only TV
unit is in production, the IR unit has been successfully demonstrated.

Either unit is then followed by the guidance adapter, in
principle a cowl providing a smooth transition to the bomb body, fitted
with a set of four strakes. The bomb body then follows, that being
either a Mk-84 2000 Ib demolition bomb or an SUU-54 submunition
dispenser, the latter version of the weapon being in late development.

The rear section of the bomb then carries the control unit,
that itself mounting the wings, control surfaces and the final
assembly,
the datalink unit. The control unit houses the autopilot, the gyros,
batteries for power and the control actuators. The datalink unit then
contains the receiver and transmitter electronics, facing aft.

The GBU-15 thus allows the user the choice of both seeker and
payload, tailored for a specific mission. Consider counter-air, night
attack. The IR seeker combined with the Mk-84 would be used to attack
the airfield, alternately the SUU-54 could be used with either cluster
bomblets against parked aircraft or mines and penetrators against
runways and taxiways.

The basic TV weapon with the Mk-84, as acquired by the RAAF,
is usually employed against hard point targets, be it bridges, command
posts, bunkers, industrial complexes, or even shipping. The SUU-54
version is even more useful, ideally suited not only to counter air but
also to anti-armour, anti-vehicular, anti-personnel and defence
suppression missions. In terms of performance, the GBU-15 is cleared
for
supersonic launch and has been extensively tested in a hostile ECM
environment, either aspect emphasising its inherent compatibility with
the F-111C and its operating environment.

The GBU-15 appears at this stage destined for a colourful
future, particularly in view of the current development of an extended
range version, with a ventrally mounted rocket sustainer which is
jettisoned after burnout. The increase in range would thus be subject
only to the burn time of the sustainer and limits of datalink range.
Whether this version eventually enters production, or is abandoned
along
the way, the basic concept of the weapon will remain - modularity. That
will ultimately ensure its survival.